Ill. Admin. Code tit. 35 § 304.105
In addition to the other requirements of this Part, effluent must not, alone or in combination with other sources, cause a violation of any applicable water quality standard. When the Agency finds that a discharge that would comply with effluent standards in this Part would cause or is causing a violation of water quality standards, the Agency must take appropriate action under Section 31 or Section 39 of the Act to require the discharge to meet whatever effluent limits are necessary to ensure compliance with the water quality standards. When such a violation is caused by the cumulative effect of more than one source, several sources may be joined in an enforcement or variance proceeding, and measures for necessary effluent reductions will be determined based on technical feasibility, economic reasonableness, and fairness to all dischargers.
Ill. Admin. Code tit. 35, § 304.105